This invention relates to positive displacement scroll pumps and in particular to means for moving scroll impellers to create a pumping action. The prior art discloses the use of two or more scroll vanes which are meshed with each other, as shown in FIG. 1, to create a pumping action. As shown in FIG. 2, the prior art method of creating a pumping action with two meshed scroll vanes is to hold one of the scroll vanes stationary while moving the other vane in a circular path which orbits about the center of the stationary vane. Neither the stationary scroll vane nor the orbiting scroll vane are allowed to rotate. Fluid is pumped between the center of the impellers and their outside circumference by the resulting cyclical variations in the sizes of the spaces between the scroll vanes and therefore the pumping action is a positive displacement action. In a centrifugal pump with a scroll like vanes, the pumping actions result entirely from the centrifugal force exerted on the fluid as the vanes rotate the fluid within the pump casing. Rotating the impeller in a centrifugal pump will cause the fluid pressure around the outside of the impeller to be higher than the fluid pressure near the center of the impeller regardless of which direction the impeller is rotated. In a scroll pump, the fluid pressure can be made higher at the outside of the impeller by orbiting the scroll impeller in one direction or the fluid pressure can be made higher at the center of the impeller by orbiting the scroll impeller in the opposite direction.
FIGS. 1 and 2 illustrate the operation of a prior art scroll pump. FIG. 1 shows six cross-sectional views of the scroll vanes in the pump and FIG. 2 is a highly simplified plan view of the over all pump. The scroll vane 21 is held stationary while the scroll vane 22 is moved in an orbital path. Because the vanes 21 and 22 are meshed with each other, the radius of the orbital path of vane 22 must be small enough to prevent it from rubbing against the stationary vane 21. FIGS. 1a through 1e illustrate the relative positions of the two scroll vanes while the orbiting vane 22 and its center 24 move about the stationary vane 21 and its center 23. From the starting position of the orbiting vane 22 as shown in FIG. 1a, the scroll orbited approximately 90 degress counter-clockwise to reach the position shown in FIG. 1b, it orbited approximately another 90 degrees counterclockwise to reach the position shown in FIG. 1c and yet another 90 degrees counterclockwise movement placed it in the position shown in FIG. 1d. Another 90 degree counterclockwise orbital movement placed the orbiting vane 22 in the position shown in FIG. 1e which is the same as its position in FIG. 1a. FIG. 1f shows the stationary vane 21 and its center 23. The dashed line and arrow 25 show the path about which the center 24 of the vane 22 orbits. Following the location and the shapes of the spaces 32, 33, 34, 35, and 38 through this same sequence of positions of the orbiting vane 22 shows how the pumping action of the two vanes occurs. As the vane 22 orbits, the space 38 at the center breaks up into the two spaces 32 and 33 which move outward and counterclockwise. After one complete orbit of the vane 22, the spaces 32 and 33 as shown in FIG. 1e are equivalent to the spaces 35 and 34, respectively, as shown in FIG. 1a. During one complete counterclockwise orbit of the vane 22, the fluid in spaces 34 and 35 will be expelled from the outside circumference of the two meshed vanes. In this manner, any fluid which is introduced through a port in the center of the scrolls will be moved about the center of the scrolls about one time and expelled at the outside circumference. Following the movements of the same spaces between the vanes through the same sequence of the five figures but in reverse order will show that if the orbiting vane 22 is orbited in a clockwise direction, the spaces between the vanes will tend to also move in a clockwise direction in from the outside of the vanes toward the center. Therefore, fluid may be pumped from the outside circumference of the scroll vanes toward their center by reversing their direction in which the scroll vane 22 orbits.
The prior art method of producing the orbiting motion of the scroll vane is illustrated in FIG. 2. The orbiting vane 22 is mounted by way of bearing 26 on the offset end of the drive shaft 31. The drive shaft 31 is supported by bearings 27 and 28 while it rotates about its axis 36. Having the scroll vane 22 thus mounted on the offset end of the rotating drive shaft 31 causes the center of the scroll vane 22 to orbit about the center of scroll vane 21. Other means, which are not illustrated in FIG. 2, are used to prevent the vane 22 from rotating as a result of torque applied to it by the shaft 31. The distance between the axis 37 of the offset portion of the shaft 31 and the axis 36 of the main portion of the shaft is referred to as the orbiting radius of the pump and is equal to the radius of the circle 25 shown in FIG. 1f.
Because the orbiting scroll vane 22 is attached to the offset portion of the rotating drive shaft 31, it places an unbalanced load on the drive shaft and on the bearings 26, 27, and 28. This unbalanced load on the bearings will cause them to wear out much faster than normal. Properly positioned counterweights near the offset end of the drive shaft could be used to balance the load on bearings 27 and 28 and to reduce the amount of vibration which is transmitted through those bearings to the pump casing. However, the counterweights would not reduce the amount of wear on bearing 26 or the amount of stress placed on the offset portion of drive shaft 31 by the orbiting vane. Therefore the offset portion of the drive shaft 31 and the bearing 26 must be made heavier than would otherwise be required and the bearing 26 must be replaced more often than would otherwise be necessary. The pumping capacity of a scroll pump may be reduced by reducing its orbiting radius below the maximum radius allowed by the scroll vanes, which are used. Changing the orbiting radius of a prior art scroll pump requires that a new drive shaft be installed on which the end attached to the orbiting scroll vane is offset by a different amount. Therefore, the pumping capacity of prior art scroll pumps cannot be changed while the pumps are in operation. In some applications, it would be desirable to be able to change the pump capacity while the pump is being driven at a constant RPM. Such a capability in a scroll pump would enable the pump to be switched to an idling mode of operation which would consume much less power from a constant RPM power source whenever less than the maximum pumping capacity is required.